The three-dimensional (“3D”) physical characteristics of surfaces of objects may be measured using various non-contact techniques and devices. Such measurements may be carried out for various reasons, including part inspection, rapid prototyping, comparison of the actual part to a CAD model of the part, reverse engineering, 3D modeling, etc. Most often, these non-contact devices utilize triangulation-based techniques for processing the raw captured data representing the surface of an object into the resulting actual measurements of the object surface.
One type of triangulation-based, non-contact device is a laser line probe (“LLP”), which includes a projector and a camera. The projector includes a light source that emits a light, typically as a line. Thus, the LLP is also known as a line scanner. The projector also includes a lens that projects the emitted light onto an object in a relatively clear (unblurred) state. The emitted light may be laser light, partially coherent light, or incoherent light. The camera includes a camera-type imaging device, such as a charge-coupled device (“CCD”) or CMOS photosensitive array. The camera also includes a camera lens that captures the pattern of light on the object surface and converts it into a relatively clear (unblurred) state on the photosensitive array. The camera is typically positioned adjacent the laser light source within the LLP device. The projector has a virtual emission point from which the line or stripe of light appears to “fan out” in an angle in one dimension and in a flat sheet in the orthogonal dimension. The camera has a camera perspective center through which rays of light from the pattern on the object appear to pass in traveling to the photosensitive array. The line segment between the virtual emission point and the camera perspective center is called the baseline, and the length of the baseline is called the baseline length.
In some cases, the LLP is shaped as a hand-held device. In other cases, it may be attached to a motorized device or fixed in position on a production line. The fan of light that strikes the surface of the object forms a relatively bright stripe of light on the object surface. The camera captures the 3D silhouette or profile of the laser stripe projected onto the object. For the case of a hand-held LLP, to cover all or some portion of an object with the line of light, the LLP is moved by the user such that the projected line stripe extends over all or at least the desired portion of the object within the LLP's field of view. That way, by moving the LLP over the object, hundreds of cross sections of the object surface are captured as 3D point clouds of raw data. Some modern LLPs can capture 60 frames, or stripes, of 3D data per second, or approximately 45,000 points of data per second. Signal processing electronics (e.g., a computer or a processor) are provided that run software which processes the 3D raw point cloud data into the resulting 3D image of the object that includes dimensional measurements as obtained by the LLP and its laser stripe and triangulation measurement process.
The image of the reflected line on the imaging device normally changes as the distance between the imaging device and the object surface changes. By knowing the baseline distance, the orientation of the projector and camera with respect to baseline, and the coordinates on the photosensitive array of the imaged pattern of light, known triangulation methods may be used to measure 3D coordinates of points on the surface of the object. That is, as the LLP is moved, the imaging device sees each projected line stripe. Any deviations on the photosensitive array from a straight line pattern may be translated into height variations on the object surface, thereby defining the object surface. In other words, the method described hereinabove digitizes the shape and position of the object within the field of view of the LLP. In this way the measured object may be checked against a CAD design model of the same object to determine any discrepancies therebetween.
Portable articulated arm coordinate measuring machines (“AACMMs”) may include a tactile probe configured to be brought into contact with an object to determine 3D coordinates of the object surface. AACMMs have found widespread use in the manufacturing or production of parts where there is a need to rapidly and accurately verify the dimensions of the part during various stages of the manufacturing or production (e.g., machining) of the part. Portable AACMMs represent a vast improvement over known stationary or fixed, cost-intensive and relatively difficult to use measurement installations, particularly in the amount of time it takes to perform dimensional measurements of relatively complex parts. Typically, a user of a portable AACMM simply guides a “hard” contact touch measurement probe (e.g., a ball) along the surface of the part or object to be measured. The measurement data are then recorded and provided to the user. In some cases, the data are provided to the user in visual form, for example, in 3D form on a computer screen. In other cases, the data are provided to the user in numeric form, for example when measuring the diameter of a hole, the text “Diameter=1.0034” is displayed on a computer screen.
An example of a prior art portable AACMM is disclosed in commonly assigned U.S. Pat. No. 5,402,582 (“the '582 patent”), which is incorporated herein by reference in its entirety. The '582 patent discloses a 3D measuring system comprised of a manually-operated AACMM having a support base on one end and a “hard” measurement probe at the other end. Commonly assigned U.S. Pat. No. 5,611,147 (“the '147 patent”), which is incorporated herein by reference in its entirety, discloses a similar AACMM. In the '147 patent, the AACMM has a number of features including an additional rotational axis at the probe end, thereby providing for an arm with either a two-two-two or a two-two-three axis configuration (the latter case being a seven axis arm).
It is generally known and accepted practice to attach a laser line probe to the probe end of an AACMM. The result is a fully integrated, portable, contact/non-contact measurement device. That is, the AACMM having an LLP attached thereto provides for both contact measurements of an object through use of the “hard” probe of the AACMM and for non-contact measurements of the object through use of the LLP's laser and imaging device. More specifically, the combination AACMM and LLP allows users to quickly inspect or reverse engineer complex and organic shapes via laser scanning, as well as to capture prismatic elements with the relatively high accuracy that contact metrology provides.
When combined as such, the AACMM and LLP may have the LLP carry out some or all of the processing of the 3D captured point cloud data using the signal processing electronics (e.g., computer or processor) within or associated with (e.g., located apart from) the AACMM. However, the LLP may have its own signal processing electronics located within the LLP or associated with the LLP (e.g., a stand-alone computer) to perform the necessary signal processing. In this case, the LLP may need to connect with a display device to view the captured data representing the object. Also, in this case the LLP may operate as a stand-alone device without the need to connect with an AACMM or similar device.
One important characteristic of any laser line probe is the dynamic range of the imaging device within the LLP. Simply put, the dynamic range of the imaging device is the range bounded on one end by the amount of relatively bright object surface portions that the imaging device is capable of accurately capturing and bounded on the other end by the amount of relatively dark object surface portions that the imaging device is capable of accurately capturing. Stated another way, the dynamic range of an imaging device is the ratio of the largest non-saturating input signal to the smallest detectable input signal. Dynamic range essentially quantifies the ability of an imaging sensor to adequately image both the highlights and the dark shadows of an object or of a larger scene. A typical real-world object or scene desired to be imaged may have a wide range of brightness values (or contrast variations) across the object surface or surfaces depending, in part, on the ambient light illuminating the object at any one point in time. For example, it is not uncommon for an object or scene to vary in brightness by 100 decibels or more.
The dynamic range required of an LLP for optimal determination of 3D coordinates of a surface is equal to the ratio of reflected optical power from the most reflective to the least reflective portions of an object surface. Dynamic range may be described as a linear ratio or, more commonly, as a logarithmic ratio in units of decibels (“dB”). The required dynamic range for a particular measurement depends partly on the material, color, and surface finish of the object surface, partly on the distances from a surface point to the projector and camera, and partly on the angles of incidence and reflectance of the projected and reflected light, respectively.
The dynamic range of an image sensor is the ratio of the largest optical energy to the smallest optical energy received by a sensing element within the image sensor. To provide a valid reading, the dynamic range received by a sensing element should be within the linear range of the sensing element, which is to say that the energy cannot be as large as to saturate or so small as to be noise limited. To perform at an optimal level, the dynamic range of the imaging device should be equal to or greater than the dynamic range required of a particular measurement. Most commercially available imaging devices, e.g., CCD's or CMOS imagers, have a dynamic range less than 100 decibels.
An LLP with a relatively low dynamic range imaging device (e.g., a CCD camera or CMOS photosensitive array) results in a reproduced image that may be too dark is some areas and/or too light (i.e., saturated) in other areas. Thus, it may be difficult, if not impossible, to accurately determine 3D coordinates with such an LLP.
As a result, many devices and techniques exist in the prior art for extending or increasing the dynamic range of imaging devices. However, these techniques and devices tend to be lacking somewhat in the amount of increase in the dynamic range of the imaging device.
While existing laser line probes are suitable for their intended purposes, what is needed is a laser line probe having an imaging device with improved (i.e., increased) high dynamic range.